How magma forms and why volcanoes erupt

Two words, one substance

Molten rock beneath the surface is magma; magma that has erupted is lava. The distinction is location, not material.^[1]

Deep within the Earth it is hot enough that some rocks slowly melt. Because the melt is lighter than the solid rock around it, it rises and collects, and some of it eventually pushes through vents and fissures to the surface. Note the word some: the mantle as a whole is solid, and melt is generated where particular conditions are met.^[1,2]

Most of that melt originates near active plate boundaries or at hot spots.^[1]

Why some eruptions explode

Explosivity depends on magma composition. Thin, runny magma lets gases escape and simply flows out. Thick, sticky magma traps its gases until pressure forces them out violently, blasting the magma apart into fragments called tephra.^[1]

Mount St. Helens erupts material ranging from basalt to rhyodacite — a compositional range that spans the runny and the viscous, and therefore the effusive and the explosive. Kīlauea, by contrast, is a basaltic shield volcano.^[1,2,3]

Three settings, not one process

Volcanoes form at divergent boundaries where plates pull apart, above subduction zones, and in plate interiors far from any boundary. These are distinct processes that happen to share an outcome.^[1]

The intraplate case is the awkward one. Most volcanism occurs near plate boundaries — but the Hawaiian Islands, entirely volcanic, formed in the middle of the Pacific more than 3,200 kilometres from the nearest boundary.^[1,2]

Wilson’s hotspot hypothesis explains this with a long-lived, exceptionally hot region beneath a moving plate, leaving an age-progressive trail of volcanoes — the Hawaiian–Emperor chain. Lithoen records it as a hypothesis, because that is what it remains.^[1,2]

Whether such hotspots are rooted in deep mantle plumes is actively contested. Tomographic imaging has resolved broad conduits beneath many hotspots reaching down to the core–mantle boundary, but the proposal is described in that same literature as the subject of vigorous debate.^[1,2]

And at subduction zones

Arc volcanism is firmly associated with subduction, but the source of its magma — slab, overlying lithosphere, or both — is disputed.^[1]

Commonly misunderstood